We host a classic road versus track battle (and a high-speed R8 family reunion) in Indianapolis. And yes, even granddad was there.

Any faithful follower of these pages knows we like to pit the fastest factory road cars against their racing counterparts to see where they'd finish in a head-to-head road race. Yes, the race cars always win, with the road car usually not even close behind. The tightest battle we've had thus far involved the 2012 Chevrolet Corvette Z06, which trailed the Le Mans-winning C6.R by a "mere" 6.28 seconds over a 1.5-mile lap at Spring Mountain ("Corvette Fever," April 2012). But we haven't lost all our faith in road cars just yet.

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For this track skirmish, we're pitting the fastest production Audi, the 560-bhp R8 GT, against the newly homologated rear-drive-only R8 Grand-Am, a race car closely related to the FIA-spec R8 LMS, an internationally successful car powered by an air-restricted V-10 that makes 480 bhp. While both are handbuilt at Quattro GmbH in Neckarsulm, Germany, how they get around the track will once again illustrate the differences between an exceptionally fast road car and a purpose-built race car. Or will it?

Our hired shoe was Florida-based racer Doug Smith, who would help us run laps around Putnam Park's 1.78-mile road course while we performed a battery of other tests on the bevy of other street R8s we had there—an embarrassment of riches, for sure.